rtnetlink: fix gcc -Wconversion warning

RTA_ALIGNTO is currently define as 4. It has to be 4U to prevent warning
for RTA_ALIGN and RTA_DATA expansions when -Wconversion gcc option is
enabled.
This follows NLMSG_ALIGNTO definition in <include/uapi/linux/netlink.h>.

Signed-off-by: Ronen Arad <ronen.arad@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>